Diesel engines have been faced with increasingly stringent emissions requirements for more than a decade. Despite substantial improvements, new emissions standards scheduled to take effect in 2007–2010 will require approximately another order of magnitude reduction in both nitrogen oxides (“NOx”) and particulate matter (“PM”) emissions. These new standards are challenging and will likely require a combination of strategies including development of advanced combustion systems to reduce engine-out emissions (i.e., emissions prior to after treatment), development of adequate, reliable and cost-effective aftertreatment systems, and improvements in the coupling between engine and aftertreatment systems operation.
Homogeneous charge compression ignition (“HCCI”) is an attractive advanced combustion process that offers potential as a high-efficiency alternative to spark ignition engines. By providing diesel-like efficiencies but with substantially lower NOx and PM emissions, HCCI also offers a low emissions alternative to diesel engines. Unlike conventional diesel combustion, HCCI does not rely on maintaining a flame front. Rather, combustion occurs as the result of spontaneous auto-ignition at multiple points throughout the volume of charge gas. This unique property of HCCI allows the combustion of very lean mixtures or mixtures that are made very dilute by the addition of combustion-product gases (e.g., by exhaust gas recirculation), resulting in low combustion temperatures that dramatically reduce NOx emissions. Also, unlike conventional diesel combustion, the charge is sufficiently well-mixed so that PM emissions are very low. Consequently, HCCI provides a low emissions alternative to conventional diesel engines.
Although the use of conventional diesel fuel or gasoline for HCCI would be desirable since these fuels are readily available, achieving acceptable HCCI with these fuels can be difficult. With diesel fuel, elevated temperatures are required before significant vaporization occurs making it difficult to form a premixed near-homogeneous charge. Second, diesel fuel has significant cool-combustion chemistry leading to rapid auto-ignition once compression temperatures exceed about 800° K. This can lead to overly advanced combustion phasing and/or require reduced compression ratios that reduce engine efficiency. Conversely, gasoline can require overly high compression ratios or various techniques to provide significant charge heating.
Currently, the power output of HCCI engines using diesel fuel or gasoline is limited to about half the that of traditional diesel engines. Extending HCCI operation to higher power outputs remains a significant challenge. This is mainly because the reaction rates with HCCI become very rapid as the fueling rate is increased causing engine knock that results in undesirable noise and reduced durability. Because high-load operation is a challenge, most HCCI concepts currently being pursued utilize HCCI only below about half load and revert to conventional spark ignition or diesel combustion for high loads. Thus, at present the advantages afforded by HCCI are limited to only part of the operating range.
Finally, it is widely recognized in the art that the charge in an HCCI engine is never fully homogeneous (in both temperature and mixture) in practical application. In some applications, such as diesel-fueled HCCI, there can substantial mixture inhomogeneities. However, the term HCCI is commonly used to refer to all cases where the charge is sufficiently premixed to achieve low NOx and PM emissions due to a quasi-volumetric, low-temperature combustion of a dilute charge. Therefore, hereinafter the term “homogeneous” in reference to homogeneous charge compression ignition (i.e., HCCI) can refer to a fuel charge that can be substantially inhomogeneous. Further, the term “fuel charge” will mean the product resulting from combining fuel plus air plus residual gases and/or combustion products or any combination thereof.